Along a busy road in the town centre where I do other shops a shop keeper asked me for a quote. 2 large windows and a door, insides once a month and the shop sign once a month plus wiping underneath the windows and generally keeping it looking nice. As I don't need the work I gave him a price a little on the high side. He said ok, and to start the following week.
About a month after I'd been doing the job the previous window cleaners turns up, puffed out chest and all indignant, and asks me why I was stealing his work. I explain and he asks me how much I was charging, so I say, and he then accuses me of under cutting him and goes into the shop to under cut me but the shop keeper wasn't interested.
The shop keeper came out to have a chat and it turns out that within reason the price wasn't the issue, this bloke wasn't always doing what he promised and he did a terrible job to boot. Also, the shop keeper said his attitude stank.
I'm not saying the op in his original posting does a bad job or has a bad attitude, but if a customer takes on a new window cleaner at a cheaper price, then maybe it's either a cheap skate customer in which case, good riddance, or there is something else not right.
Most customers are pretty loyal to their own window cleaner and I've found that keeping customers is often more about good relationships than anything else.